1. Field of the Invention
The present invention relates to the purification of mercaptobenzothiazole and particularly to the purification of mercaptobenzothiazole prepared according to known processes.
2. Description of the Prior Art
Mercaptobenzothiazole is known in the elastomer conversion industry for its utilization as a vulcanization accelerator. It is likewise an important raw material in the synthesis of improved vulcanization accelerators which are adapted to the particular problems posed by the manufacture of articles as different as, for example, pneumatic tires, electric cables, shoe soles, and insulation joints. It also enters widely into the synthesis of plant protection compounds.
Most of the known processes of manufacture make use of the reaction, in appropriate proportions and at high temperature and high pressure, of aniline, sulfur, and carbon disulfide. Others make use of the reaction of thiocarbanilide, carbon disulfide and sulfur (U.S. Pat. No. 1,712,968, issued May 14, 1929), or of the reaction of orthochloronitrobenzene, hydrogen sulfide or an alkaline sulfide, and carbon disulfide (U.S. Pat. No. 1,960,205, issued May 22, 1934; Polish Pat. No. 86,988, issued Dec. 15, 1976); or to the reaction of benzothiazole and sulfur (German Pat. No. 2,551,060 issued May 6, 1976). The reaction product obtained under such conditions can never be utilized directly as it is. It contains, in fact, raw materials which have not reacted, for example, aniline, etc.; and by-products and intermediates such as benzothiazole or anilinobenzothiazole. A careful purification of the crude reaction product is necessary.
Numerous purification processes have been proposed in the past. These processes fundamentally make use of two techniques which differ mainly in the concentrations, order of use, and nature of the recommended reagents, and in the temperatures of treatment.
The basic steps of the principle of the first technique are as follows:
1. Solution of the reaction product in an alkaline medium (ammonium hydroxide, sodium hydroxide, lime), optionally preceded by a treatment in a mineral acid medium;
2. Separation of the insoluble impurities by filtration;
3. Separation of the soluble impurities after they have been made insoluble by oxidation and/or extraction by means of a solvent; and
4. Precipitation of the mercaptobenzothiazole by the action of a mineral acid.
U.S. Pat. Nos. 1,631,871; 2,658,864; 2,730,528; and 3,818,025 and French Pat. No. 2,135,807 illustrate the application of the whole, or a part, of such a process.
In the second technique, the impurities are extracted by treatment of the reaction product with carbon disulfide or an emulsion of carbon disulfide and water.
U.S. Pat. Nos. 2,090,233; 3,030,373; and 3,031,073 illustrate this embodiment.
Although they are industrially exploited at present, these processes are not satisfactory and each present one or more of the following disadvantages:
1. Difficult recovery of the unreacted raw materials, which are economically very important to recycle (aniline in particular);
2. Necessity of operation at low concentrations to favor the precipitation of impurities, with the consequential use of apparatus of large dimensions;
3. Losses, by chemical degradation, of mercaptobenzothiazole during the course of the oxidation reactions intended to render the impurities insoluble in an alkaline medium;
4. Losses, by solution in the carbon disulfide, of mercaptobenzothiazole, or inevitable recycling of part of the impurities if the quantity of carbon disulfide is limited; and
5. Finally, and this is probably the major disadvantage of these processes, the necessity of having to treat large volumes of aqueous effluents, containing heavy pollutant loads, before their release. These treatments are difficult and costly. The oxidation processes generally applied do not lead to the desired simple molecules of nitrogen, carbon dioxide and sulfur dioxide, but to an unacceptable level of soluble molecules which are not degraded by a complementary biological treatment, heavily adding to the cost per unit of production.